This invention relates to natural and synthetic rubbers and, in particular, such rubbers which have been extended with oil. Specifically, the invention relates to oil-extended rubbers, and to compositions thereof containing a flame retardant which include viscosity index modifiers and to the use of such modifiers in enhancing the flame retardant characteristics of oil-extended rubber.
Prior to the introduction of synthetic rubber during World War II, vegetable and mineral oils and similar materials had been used in small amounts in the processing of natural rubbers. In this particular application, a small amount of the oil was added to the rubber in its latex form as a softening agent to reduce power consumed in the mixing and mastication of the rubber mass.
The short supply of rubber and the introduction of synthetic rubbers during World War II brought a number of new problems to the rubber industry and necessitated much work before the synthetic rubbers could be used successfully. Many of these problems involved modifiers and extenders for the new rubbers and for formulations involving admixtures of the new rubbers with natural rubber.
From the standpoint of processing ease, it was determined that the addition of oils, such as those previously employed as softeners or plasticizers, reduced the Mooney viscosity to the desired extent and had no detrimental effect on the physical properties of the product. It was also found that these processing oils could be used in amounts far in excess of the amounts normally used in softening and plasticizing to replace a certain amount of the synthetic polymer and thus "extend" the polymer.
The use of oils of petroleum origin as extenders for natural rubber and synthetic elastomers was first proposed in 1936. The first product introduced for this purpose was a petroleum oil recovered from the acid sludge produced in the treatment of petroleum oils with sulfuric acid. This material was used commercially as a rubber extender just before and during World War II. A number of petroleum oil extenders have since appeared on the market. Naphthenic oils as well as highly aromatic oils, such as extracts from the solvent extraction of lubricating oils, are used commercially.
It has been recently found that such extending oils, while of inestimable value to the rubber industry, contribute to the flammability of the rubber composition. Thus, at higher processing temperatures the oil has a low flash point and causes quick ignition of the rubber composition. This problem is especially severe in sponge rubber systems.
The flammability of oil-extended rubber compositions, even those containing conventional flame retardants, has caused increased concern with the advent of greater amounts of such rubber compositions being used in upholstery and carpeting compositions. Accordingly, there is a need for imparting flame retardant characteristics to oil-extended rubber compositions.